Electrochemical assays for determining the concentration of enzymes or their substrates in complex liquid mixtures have been developed. For example, electrochemical sensor strips have been developed for the detection of blood glucose levels. Electrochemical sensor strips generally include an electrochemical cell in which there is a working electrode and a reference electrodes. The potential of the working electrode typically is kept at a constant value relative to that of the reference electrode.
Electrochemical sensor strips are also used in the chemical industry and food industry, to analyze complex mixtures. Electrochemical sensors are useful in biomedical research, where they can function as invasive probes, and for external testing (i.e., testing of blood obtained by a needle and syringe, or a lance).
Typical electrochemical sensors for blood analysis measure the amount of analyte in a blood sample by using a working electrode coated with a layer containing an enzyme and a redox mediator and a reference electrode. When the electrodes contact a liquid sample containing a species for which the enzyme is catalytically active, the redox mediator transfers electrons in the catalyzed reaction. When a voltage is applied across the electrodes, a response current results from the reduction or oxidation of the redox mediator at the electrodes. The response current is proportional to the concentration of the substrate. Some sensors include a dummy electrode coated with a layer containing the redox mediator and lacking the enzyme that improves the accuracy and precision of the measurements.
Thin layer electrochemical sensors that have electrodes confined in a covered region of the sensor generally have a venting system for releasing air from the confined electrode region upon displacement by the sample. In a two-mesh electrode construction, the sample wicks along the primary layer and the displaced air is vented from the second mesh layer. See, e.g., U.S. Pat. No. 5,628,890.
Reducing the total sample volume necessary to produce a precise and accurate analyte concentration reading by an electrochemical sensor would enhance user convenience. Reducing sample volume is particularly desirable in a blood analysis, because pain, messiness, and time required to stop bleeding generally increase as blood sample size increases.
Although sample size reduction is desirable, various constraints severely limit the ways in which this can be accomplished. Constraints include the following. Sample volume must be sufficient to cover the entire electrode area. Reducing electrode area changes the electrode response current, thereby rendering an electrode strip incompatible with a given meter. Where the sample is not applied directly to an electrochemical cell, the total volume required includes the volume necessary to cover a sample loading zone, and a path to the electrodes, as well as the electrode area. The sample loading zone must be easily visible, even to diabetics with impaired vision.